Headcrabs are fairly small creatures, roughly the size of a pumpkin, which crawl on four legs, and have enough jumping power to leap up to 10 feet (3 meters) in any direction. Their underbelly is almost completely covered by a large, lipless mouth with a hidden "beak" used for feeding and latching onto the skulls of victims... [They] are a parasitic species, attaching to the host's head and... [a]fter breaking through the skull, the Headcrab proceeds to take over its host's motor functions through some unknown biological process, keeping its host alive but helpless through the entire process.